I get this error from time to time when exporting to file. It also says "the desired binary x3g file was not created." I am using el Capitan OSX. I read elsewhere to replace my GPX pluggin but mine is dated Nov 2015. Any idea?
Well you didn't really provide any additional information for us to help. I have the exact same OS and that file runs fine for me. So use the terminal. Run an "ls" to prove you're in the right directory. Check the permissions on the file to make sure you didn't damage those. Try to run it. I can't imagine there would be no output at all. You're either going to get an error running it, or you will see the normal GPX usage instructions.
It works fine for others on el capitan, so if you want help, you're going to have to provide more information